1. Field of the Invention
The present invention relates to an improved rigid filter plate for sludge dewatering and to an improved method for making such a rigid filter plate.
Sludge treatment processes provide means to reduce the volume and moisture content of waste water sludge to yield an easily handleable, relatively dry sludge cake. Efforts directed toward improving existing sludge dewatering technology have not entirely overcome the inherent drawbacks of current filtration means. Many current filtration systems utilize loose sand or other nonbonded particulate matter as the filtering element. The filtering layer of loose sand does not possess the strength or durability of an adhesively bonded filtering element. Even those filter plates characterized by rigid surfaces are generally unable to withstand mechanized sludge cake removal by heavy equipment traversing the filtering element without flaking, chipping, or breaking of the filter plate. In addition, most filter plates capable of use with vacuum filtration require frequent backwashing to maintain their efficacy.
The effectiveness of the sludge treatment process depends greatly upon the efficiency of the filtering medium. Efficiency comprises two separate but related functions of the filtering medium. First, the filter medium should permit maximum speed in the dewatering process; that is, permit rapid reduction of volume and moisture content of the sludge to yield a handleable, relatively dry sludge cake. Second, the filtering medium must product a clear, clean, relatively solid-free effluent during the rapid dewatering process.
Clearly, filtering medium characteristics and properties profoundly affect the overall efficiency and cost of sludge treatment facilities. There is a need in the sludge treatment industry for a permanent filtering medium that provides fast, efficient dewatering and yet retains its structural integrity during repeated mechanized removal of the sludge cake. Many known filter media variously suffer from the disadvantage of meeting one of these requirements but not all of them, or are effective with a few types of sludges but not with a wide variety of sludge types.
2. Description of the Prior Art
U.S. patent application Ser. No. 202,241, filed Oct. 30, 1980 by Morris M. Riise and assigned to applicant's assignee's licensor, and its precursor applications, teach the use of a rigid filter plate in a sludge dewatering system. The filter plate disclosed therein comprises a monolithic multilayer filter plate. The term "monolithic" is used to denote a rigid unified structure. The applications teach using an upper layer filtering element composed of sharp pointed particulate matter roughly uniform in size. The sharp, upwardly extending points on the upper surface of the filtering element are emphasized as desirable to spear flocculated sludge masses. The filter plate provides a rigid structure through the application of epoxy to bind the plate's particle components. However, it has been discovered that filter plates prepared in accordance with the teachings of these applications develop bare spots on the upper surface of the filtering element with repeated use, and that either scraping of the surface with front end loaders or hosing off of the surface under very high pressure to clean the surface results in chipping of the filter plates.
U.S. Pat. No. 4,190,534 to Wyatt teaches the use of a multilayer filter bed for filtering and removing sludge materials suspended in the waste water by-product of steel pickling operations. The patentee describes using sharp pointed particles to form the exposed surface of the filter. These sharp pointed particles spear the larger sludge masses during filtering. Neither the particulate material constituting each layer of this filter nor the layers themselves are bonded by an adhesive bonding agent. Such a filter does not possess enough rigidity, strength, or durability to permit mechanized removal of sludge cakes by heavy equipment such as front end loaders. Also, cleaning the filtering element involves removal of its exposed surface. The patentee gives no indication regarding the suitability of the filtering medium for those rapid dewatering systems which employ vacuum filtration means.
A technical bulletin from Filtros, Inc. discusses use of ceramically bonded fused alumina for filter underdrain plates. Although the bulletin illustrates the use of porous ceramically bonded underdrain plates in a water filtering system, the main filtering element is loose sand. This system bears little relation to the present invention.
U.S. Pat. No. 3,011,643 to McCoy teaches a sand filter apparatus for swimming pool applications utilizing a bonded lower support structure for the single layer of filtering sand. This filter is not used for sludge dewatering. It employs a single layer filtering medium composed of loose sand. There is no teaching or suggestion of vacuum filtration compatible with the disclosed filter. The single layer filtering medium is not bonded by an adhesive bonding agent nor does it possess an exposed surface of sharp pointed particles. U.S. Pat. Nos. 3,847,808; 3,771,655; and 3,615,019 are of similar scope and are drawn from related subject matter areas but are less pertinent.
U.S. Pat. No. 3,166,615 to Farrell teaches a method of manufacturing a bonded multilayer porous structure capable of use as a mold for vacuum forming of pulp products. The disclosed invention is not related to sludge dewatering. The patentee does not disclose the use of sharp pointed particles on the exposed surface of the filtering medium or an underlying support structure comprising particulate material bonded by an adhesive bonding agent. U.S. Pat. Nos. 3,056,704; 2,293,099; 2,155,016; and 1,844,528 are drawn from the art related to bonding processes for minute particulate matter but are less pertinent.